3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is a synthetic compound structurally related to stimulants and hallucinogens. In the United States, in 2001, an estimated 3.6% of Americans aged 12 or older had tried ecstasy at least once in their lifetime (NHSDA). The frequency of recorded incidents associated with its acute toxicity is high. MDMA-related hospital emergency room incidents increased from 253 in 1994 to 4,511 in 2000 (SAMHSA Drug Abuse Warning Network). The potential neurodegeneration in subjects consuming MDMA is the most worrisome issue of its misuse. Although there is a growing consensus that MDMA is toxic to humans, the mechanisms underlying its toxic effects remain to be elucidated. Among the theories for the neurotoxic and acute toxic effects of MDMA, clinical and experimental data from animal models support the view that systemic metabolism contributes significantly to neurotoxicity. MDMA exhibits a non-linear pharmacokinetics in humans, a phenomenon related to its ability to inhibit its own metabolism (CYP2D6 is the enzyme involved) via the formation of an inactivating enzyme-metabolite complex. MDMA metabolites, which can generate oxygen free radicals, oxidative stress, and membrane damage, may contribute for the long-term MDMA-induced neurodegeneration. The key enzymes involved in MDMA metabolism are polymorphic (CYP2D6, COMT), exhibiting significant inter-individual and gender differences, which may contribute to differences in MDMA pharmacology and susceptibility to toxicity. A project is proposed that will study the metabolic factors involved in the toxicity of MDMA. To investigate the acute effects of MDMA, the project proposes to study the involvement of CYP2D6, CYP3A4 and CYP1A2 and COMT in MDMA metabolism. Particularly it is proposed to investigate the autoinhibition of CYP2D6 by MDMA and the contribution of other alternative CYP catalysed pathways. A clinical trial is proposed where MDMA will be administered in conjunction with probe drugs for CYP2D6 and CYP3A4 (dextromethorphan) and for CYP1A2 (caffeine). A separate trial is proposed whereby MDMA will be administered to individuals possessing poor or ultrarapid metabolism for CYP2D6. Physiological, and subjective effects, psychomotor performance, hormones and pharmacokinetics will be evaluated. Several animal studies in rats will compliment studies in humans. The development of neurotoxicity will be assessed co-administering MDMA with probe drugs, inhibitors of CYP2D6 and COMT activities. It has been postulated that the production of reactive oxygen species (ROS) is involved in the development of MDMA neurotoxicity. The metabolism of MDMA may contribute to the production of ROS through the formation of thioether-conjugates of catechol metabolites, whose formation in vivo in rats has been demonstrated. Transporters that regulate the efflux from hepatocytes and the passage through the blood brain barrier of conjugates such as MRP1 may play a role in MDMA neurotoxicity. Investigating the pharmacological implications of Phase III transporters would demonstrate the contribution of MDMA metabolism to mid-long term toxicity. A study will compare MDMA toxicity when co-adminsitered with the MRP1 inhibitor probenecid.